Observing Overheads

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Current estimates are that the overheads associated with each
new science target (for target acquisition, telescope, WFS and
instrument re-configuration etc) depend upon the guide configuration
chosen for the observation.

Setup times for various observation types are:

Mode

Setup times

NIFS+PWFS2 (non-AO)

11 mins

NIFS+Altair-NGS

11 mins

NIFS+Altair-LGS

25 mins

NIFS OIWFS

add 5 mins

NIFS Coronagraphy

add 4 mins

Long observations may be split over several nights to better accommodate them in the queue. Please allow for one acquisition for every 1.5 - 2 hours of observing when calculating the time required for your project.

Acquiring with the coronograph requires some additional iterations during acquisition to ensure an accurate centering behind the occulting disk. Likewise, acquiring the OIWFS star incurs an additional overhead of approximately 5 minutes. These overheads are in addition to the baseline set-up time for a given guide configuration. For example, the assumed overhead for an NGS-AO coronographic observation including an OIWFS star (recommended) is 11+4+5 = 20 minutes.

Dithering overheads: 30s per dither step

For the small 3" x 3" FOV of NIFS, it will be necessary to dither the
telescope off the science source to acquire a sky spectrum for nearly
all observations (see examples in the OT Library).

For
automated sequences of exposures (dither patterns) the estimated
on-source efficiency is ~40%; (i.e. 60%; of the elapsed time is used
for detector readout, telescope offsetting, WFS re-acquisition etc).
This estimate is quite realistic for a typical
target->sky->target->sky... observations.

Very short
exposures will have lower efficiency because of the fixed overhead per
image (up to 30 seconds for dithers). Longer exposures will have higher
efficiency.

Readout overheads

Observations in the low noise
readout modes incur higher overheads. Standard "bright object" readout requires 5 seconds per
image, medium read mode is 21 seconds per image, and the "faint object"
readmode, which provides the lowest 6e- readnoise, requires 85 seconds
to read out each frame. Also, readout overheads accumulate for every
coadd, so a "faint object" observation that has 8 coadds will have 8 x 85
= 680 seconds of readout overhead! This is not recommended. For optimal
efficiency, it is best to chose a readmode based on the brightness of
your target and the length of the individual exposures.

Readmode

read noise

readout time (per coadd)

Bright source

18 e-

5.3 sec

Medium source

9 e-

21.2 sec

Faint source

6 e-

84.9 sec

Grating change overhead: 65s

A new OT for 2006B CfP should have all the above NIFS overheads
properly handled, then "Timeline" in the "Basic Sequence Component"
should be close to the actual exection time (i.e., wall clock time).

The
current OT assumes a fixed overhead (0.17 min = 10.2 sec) for all
telescope & instrument setups. Actual times for some of these
setups are quite off from the fixed overhead of 10.2 sec. Two main
discrepancies are offsets (or dithering) and grating changes. For each
dither position, overhead is ~30 sec (instead of 10.2 sec) and a
grating change takes typically ~65 sec.

The "Planned
Observing Time" value from the current OT includes only actual
exposures plus the initial target setup time. It does not include any
telescope and/or instrument overheads after the initial target setup. A
true observing time can be obtained by adding up all items which appear
in the "Timeline". To see this "Timeline", click sequence ->
TimeLine.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in five participant countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, the Brazilian Ministério da Ciência, Tecnologia e Inovação and the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.